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AIRBORNE KINEMATIC GPS POSITIONING FOR PHOTOGRAMMETRY THE DETERMINATION OF THE CAMERA EXPOSURE ... - page 2 / 11

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RELATIONSHIP OF THE GPS ANTENNA PHASE CENTER TO EXPOSURE STATION

The GPS receiver collects the carrier phase and

pseudo-range information pertaining to aircraft throughout the photo mission.

the trajectory of the The raw data is post

processed into trajectory information consisting of GPS

time-tagged geocentric positions for center. This position is correlated station through time and orientation

the GPS antenna phase to the camera exposure of the spatial offsets

exposure

must be

computed

by

exposure

station

position

can

between two origins, earth center and photo center. rarely coincide with the times at which the antenna

Exposures positions

are recorded.

Therefore,

of

the antenna position at

interpolation.

The

then

be

determined

the time camera using an

orthogonal

three-dimensional

transformation

incorporating

the

spatial

offsets

between

the

entrance

node

of

the

lens

system

and GPS antenna phase center and a elements of exterior orientation.

priori estimates for the These a priori estimates

may be refined during aerotriangulation and more accurate

exposure stations aerotriangulation

computed solution

after each iteration of the

(Lucas

1989).

The

error

of

the

individual component observations can be process to yield a variance-covariance exposure station.

propagated during this matrix for the camera

INTERPOLATION ACCURACY AS A FUNCTION OF GPS SAMPLING RATE

Since it is not presently practical to control the GPS collection by the exposure event, the antenna phase center position at the time of exposure must be computed by interpolating the aircraft trajectory after the appropriate timing biases have been removed from the observed exposure time.

The exposure station position accuracy is a function of the kinematic positioning accuracy (1 or 2 cm relative accuracy for most geodetic quality GPS receivers), sampling rate and

interpolation model. sampling rates in part

GPS receiver manufactures have increased to enhance the accuracy of interpolation.

The inclination would be to sample the GPS receiver at as high a frequency as possible to minimize the time difference between

GPS the

epochs and

exposure

processing

burden.

times.

A

more

This practice also increases

practical

solution

would

be

to

select a exposure

sampling rate and interpolation model station accuracy commensurate with the

which final

yield mapping

product effort.

while at the same time minimizing post processing Operational experience gained by NOAA has demonstrated

that a 1 hertz GPS sample is as 1:10,000 when using the

adequate for photo scales as large interpolation model discussed in

this paper.

Higher sampling rates or more different

interpolation

models

may

be

necessary

for

low

altitude

(larger

scale)

photography

to

accommodate

aircraft

trajectories

influenced by short period turbulence. following analysis of the interpolation

To this end, the process is presented.

The GPS signals are generally sampled on a nearly uniform time interval affected only by a very small (one usec/sec) (King

and

Durboraw

1988)

drift

in

the

receiver

clock.

The

signals,

subsequent to being clock drift removed

post-processed generally have receiver and are time shifted so that all antenna

positions are equally spaced in time.

The interpolation

becomes

simplified

since

the

data

points

are

evenly

spaced

in

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